1. Field of the Invention
The present invention relates to an optical device for use in a reading apparatus of an optical information recording medium such as an optical disc, and more particularly, it relates to an optical device which is suitable for a compatible reproduction system of a DVD (known as xe2x80x9cDigital Versatile Discxe2x80x9d or xe2x80x9cDigital Video Discxe2x80x9d) and a compact disc-write once (CD-R) and which can be miniaturized.
2. Description of the Prior Art
In place of a CD as a household optical disc system which has already generally spread, a higher-density DVD system has been proposed/commercialized, and has started to spread in recent years. In a DVD player which is a reproduction apparatus, CD compatible reproduction becomes essential in order to avoid the redundancy or operation intricacy of the apparatus. Moreover, also with respect to a compact disc-write once (CD-R) which can be reproduced by the CD player, a compatible reproduction function is similarly requested. Therefore, a technique for reproducing various normal discs has been developed, and the simplification and cost cutting of a constitution for realizing the technique become themes.
Above all, in the aforementioned CD-R, the reflectance of a recording medium has a large dependence on wavelength, and hence, a laser light source of a 780 nm band different from a 650 nm band for a DVD is essential, and a pickup optical system having a built-in light source of two wavelengths is necessary.
Accordingly, there have been developed a device obtained by mechanically coupling two conventional and independent pickups, a device obtained by independently attaching received/emitted light integration elements for wavelengths, synthesizing them on one optical axis by a dichroic prism, and sharing a partial optical system such as an objective lens, and the like. In addition, another device has been proposed which can be obtained by receiving, in one package, two semiconductor laser chips different in wavelength from each other, setting other components to be independent of one another but setting the optical axis to be common.
On the other hand, with a request for cost down and small size, an attempt to integrate an optical circuit for an optical pickup has also been developed. For example, a device has been developed by unifying a semiconductor laser (LD), a photodetector (PD) and a holographic optical element (HOE), and has been applied to a CD and DVD. Moreover, in a society, further integration with two wavelengths has also been proposed (e.g., ISOM""98 Technical Digest pp22 and subsequent pages, Tu-D-01).
As described in the above document, in an integrated device in which the semiconductor laser can be disposed very close to the photodetector, it is easily possible to dispose a light receiving portion of a diffracted light by the holographic optical element and a light emitting point of the semiconductor laser in a substantially conjugate position. Therefore, focus error detection can be realized by a complementary spot size detection method (SSD method) in which xc2x11st order diffracted lights by the holographic optical element are both utilized. This method is advantageous as compared with another practical xe2x80x9cknife edge methodxe2x80x9d in that strict position adjustment of the holographic optical element is not necessarily required, it is unnecessary to discard one of the xc2x11st order diffracted lights and high efficiency is obtained.
FIGS. 1A and 1B are explanatory views showing the focus error detection by the aforementioned spot size detection (SSD) method (Japanese Patent Application Laid-Open No. 101417/1993). More specifically, FIG. 1A is a schematic side view of an apparatus for performing the focus error detection, and FIG. 1B is a schematic plan view of a photodiode for detecting the diffracted light in the apparatus.
As shown in FIG. 1A, in this focus error detection apparatus, a reflected light reflected by an optical disc 357 is transmitted through an objective lens 356 and separated into a pair of conjugate lights b1, b1xe2x80x2 by a holographic optical element 355. Here, the holographic optical element 355 is constituted in such a manner that the conjugate light b1 is focused above a light receiving element substrate 350, and the conjugate light b1xe2x80x2 is focused below the substrate 350.
Moreover, as shown in FIG. 1B, the respective conjugate lights b1, b1xe2x80x2 are received by photo detection diodes 352 and 353 disposed in the light receiving element substrate 350. The photo detection diodes 352 and 353 are divided into three areas 352a, 352b, 352c and 353a, 353b, 353c in Y direction crossing at right angles to X direction in which the conjugate lights b1 and b1xe2x80x2 are separated.
By this constitution a laser light focus error signal FE to the optical disc 357 is given by the following equation when outputs of the light receiving areas 352a, 352b, 352c are w1, w2, w3, respectively, and outputs of the light receiving areas 353a, 353b, 353c are w4, w5, w6, respectively:
FE=(w1+w3+w5)xe2x88x92(w2+w4+w6)xe2x80x83xe2x80x83(1)
Specifically, when a laser light emitted from a laser light source 351 and raised by a raising mirror 354 is incident upon the optical disc 357 via the objective lens 356, and a focus of the laser light is adjusted with respect to the disc 357, a spot S1 on the photo detection diode 352 becomes the same in size as a spot S2 on the photo detection diode 353, and the focus error signal FE of the equation (1) turns to zero. On the other hand, when the focus of the laser light deviates from the optical disc 357, the spot S1 on the photo detection diode 352 becomes different in size from the spot S2 on the photo detection diode 353, and the focus error signal FE of the equation (1) indicates a positive or negative value other than zero. Therefore, a polarity of the focus error signal FE is reversed before and after a focusing point. Therefore, by detecting the focus error signal FE, focus adjustment of the laser light with respect to the optical disc 357 can be performed.
Additionally, when the focus error detection by the spot size detection method and the 2-wavelength optical system are to be both established, the dependence of a diffraction angle by the holographic optical element on the wavelength raises a problem.
Specifically, in a diffraction grating, the diffraction angle and other characteristics are determined by a mathematical relation between a period structure and light wavelength, and therefore the diffraction angle largely changes with respect to different wavelengths. More specifically, in the xe2x80x9cspot size detection methodxe2x80x9d as the focus error detection method suitable for the integrated device using the holographic optical element, it is essential to dispose a photodetector light receiving surface for detecting the holographic optical element diffracted light in the very vicinity of the conjugate point of the semiconductor laser light emitting point. However, when lights with different wavelengths are incident upon the same holographic optical element, an optimum photodetector light receiving surface position largely differs by the characteristic change. Therefore, it has been difficult to integrate the semiconductor laser and photodetector on the same photodetector substrate. Moreover, also with respect to aberration correction or the like for optimizing a holographic optical element lens action, it has been difficult to derive a compatible solution.
For example, in the aforementioned 2-wavelength integrated device (ISOM ""98 Technical Digest pp22 and subsequent pages, Tu-D-01), only one of the xc2x11st order diffracted lights is used for each wavelength, and the complementary constitution is not realized.
Moreover, in a DVD-RAM, tracking error detection of a differential push-pull (DPP) system is preferable, but in an integrated pickup using hologram or the like to satisfy small size, high rate and low cost, it has been difficult to realize the tracking error detection of the DPP method for the DVD-RAM without causing cost up or performance deterioration.
Therefore, an object of the present invention is to provide an optical device which realizes complementary focus error detection with respect to two wavelengths in an optical system using lights of two wavelengths.
In order to achieve the aforementioned object, according to an aspect of the present invention, there is provided an optical device for reading information from an information recording medium, comprising: a first light source for outputting a light of a first wavelength; a second light source for outputting a light of a second wavelength; a holographic optical element having a first diffraction area and a second diffraction area for diffracting the lights of the first and second wavelengths; and a light receiving element substrate provided with a first light receiving element and a second light receiving element for receiving a diffracted light from the holographic optical element, wherein the first diffraction area and the second diffraction area have grating arrangements whose grating axis directions are parallel to each other and whose grating pitches are different from each other, light emitting points of the first and second light sources are apart from each other by a predetermined distance in a direction crossing at right angles to the grating axis, the grating pitches of the first diffraction area and the second diffraction area are determined in such a manner that: when a distance (L11;L12) between an incident position of the diffracted light of the first wavelength to the surface of the light receiving element substrate by the first diffraction area or the second diffraction area, and an optical axis determined by a 0th order transmitted light of the first wavelength is a first distance, and a distance (L21;L22) between an incident position of the diffracted light of the second wavelength to the surface of the light receiving element substrate by the same diffraction area, and the optical axis determined by the 0th order transmitted light of the second wavelength is a second distance, a difference (|L11xe2x88x92L21|; |L12xe2x88x92L22|) between the first distance and the second distance becomes substantially equal to an interval between the light emitting points of the first and second light sources; and an interval (|L11xe2x88x92L12|; |L21xe2x88x92L22|) between the incident position of the diffracted light of the first or second wavelength to the light receiving element substrate surface by the first diffraction area, and the incident position of the diffracted light of the same wavelength to the light receiving element substrate surface by the second diffraction area becomes substantially equal to the interval between the light emitting points, the diffracted lights of the first wavelength and the second wavelength by the first diffraction area are converged to much the same first position on the light receiving element substrate, and the diffracted lights of the first wavelength and the second wavelength by the second diffraction area are converged to substantially the same second position on the light receiving element substrate, and the first and second light receiving elements are disposed in the first and second positions, respectively.
In a preferred embodiment of the present invention, a focus error signal is obtained on the basis of signals from the first light receiving element and the second light receiving element.
In a preferred embodiment of the present invention, the diffracted lights to the first and second positions are both +1st order diffracted lights by the first diffraction area and the second diffraction area, and the interval between the light emitting points of the first and second light sources and the grating pitches of the first diffraction area and the second diffraction area are set in such a manner that xe2x88x921st order diffracted lights of the first wavelength and the second wavelength by the first diffraction area and the second diffraction area are converged to third, fourth, fifth, sixth positions apart from one another by a predetermined interval capable of receiving the lights in independent light receiving areas not superposed to one another on the light receiving element substrate.
In a preferred embodiment of the present invention, the xe2x88x921st order diffracted lights of the first wavelength by the first diffraction area and the second diffraction area are converged to the third and fourth positions, the xe2x88x921st order diffracted lights of the second wavelength by the first diffraction area and the second diffraction area are converged to the fifth and sixth positions, a tracking error signal for the first wavelength is obtained on the basis of detection signals from the light receiving elements disposed in the third and fourth positions, and a tracking error signal for the second wavelength is obtained on the basis of the signals from the light receiving elements disposed in the fifth and sixth positions or the signals from the light receiving elements disposed on both side areas opposite to each other in a grating axis direction of the fifth or sixth position.
In a preferred embodiment of the present invention, when the information recording medium is a CD-R, the tracking error signal for the second wavelength is obtained on the basis of the signals from the light receiving elements disposed in the fifth and sixth positions.
Moreover, in order to achieve the aforementioned object, according to another aspect of the present invention, there is provided an optical device for reading information from an information recording medium, comprising: a first light source for outputting a light of a first wavelength; a second light source for outputting a light of a second wavelength; a holographic optical element having a first diffraction area and a second diffraction area for diffracting the lights of the first and second wavelengths; and a light receiving element substrate provided with a first light receiving element and a second light receiving element for receiving a diffracted light from the holographic optical element, wherein in the first diffraction area and the second diffraction area, grating pitches are identical with each other, grating axis directions are different from each other by a predetermined angle of 30xc2x0 or less, and light emitting points of the first and second light sources are apart from each other by a predetermined distance in a direction substantially crossing at right angles to the grating axis direction, the grating pitches of the first diffraction area and the second diffraction area are determined in such a manner that: when a distance between an incident position of the diffracted light of the first wavelength to the surface of the light receiving element substrate by the first diffraction area or the second diffraction area, and an optical axis determined by a 0th order transmitted light of the first wavelength is a first distance, and a distance between an incident position of the diffracted light of the second wavelength to the surface of the light receiving element substrate by the same diffraction area, and the optical axis determined by the 0th order transmitted light of the second wavelength is a second distance, a difference between the first distance and the second distance substantially becomes equal to an interval between the light emitting points of the first and second light sources, directions of the first diffraction area and the second diffraction area are determined in such a manner that: the diffracted lights of the first wavelength and the second wavelength by the first diffraction area are converged to much the same first position on the light receiving element substrate; and the diffracted lights of the first wavelength and the second wavelength by the second diffraction area are converged to substantially the same second position apart from the first position by a predetermined distance in a direction crossing at right angles to the light emitting point apart direction on the light receiving element substrate, and the first and second light receiving elements are disposed in the first and second positions, respectively.
In a preferred embodiment of the present invention, a focus error signal is obtained on the basis of signals from the first light receiving element and the second light receiving element.
In a preferred embodiment of the present invention, the diffracted lights to the first and second positions are both +1st order diffracted lights by the first diffraction area and the second diffraction area, a tracking error signal of the first wavelength light is obtained on the basis of signals from the light receiving elements disposed in the third and fourth positions in which the xe2x88x921st order diffracted lights of the first wavelength by the first diffraction area and the second diffraction area are converged on the light receiving element substrate, and a tracking error signal of the second wavelength light is obtained on the basis of the signals from the light receiving elements disposed in the fifth and sixth positions in which the xe2x88x921st order diffracted light of the second wavelength by the first diffraction area or the second diffraction area is converged on the light receiving element substrate or the signals from the light receiving elements disposed on both side areas opposite to each other in a grating axis direction of the fifth position or the sixth position.
In a preferred embodiment of the present invention, when the information recording medium is a CD-R, the tracking error signal for the second wavelength is obtained on the basis of the signals from the light receiving elements disposed in the fifth and sixth positions.
In a preferred embodiment of the present invention, the first light receiving element and the second light receiving element are divided into a plurality of sub areas by a plurality of division lines, and the focus error signal is obtained on the basis of the signals from the plurality of sub areas.
In a preferred embodiment of the present invention, when a point at which the 0th order transmitted light intersects the light receiving element substrate is P, an angle formed by a radial axis defined in a radial direction crossing at right angles to a track of the information recording medium and a straight line connecting the first or second position to the intersection point P is xcex81, and an angle formed by the radial axis and the plurality of division lines is xcex82, a relation of 0 less than xcex82 less than xcex81 is satisfied.
Moreover, in order to achieve the aforementioned object, according to still another aspect of the present invention, there is provided an optical device for using a laser light having a predetermined wavelength to read information from an information recording medium, comprising: a laser light source for generating the laser light; a light receiving element substrate provided with a plurality of light receiving areas in the same plane; a 3-beam generating diffraction grating for branching the laser light from the laser light source to three emitted lights; and a holographic optical element, divided into at least a first area and a second area in the same plane, for branching a reflected light from the information recording medium and turning the light to the light receiving element substrate, wherein diffraction axes of the first area and the second area are formed in such a manner that a diffraction axis direction of xc2x11st order diffracted lights by the first area and the diffraction axis direction of the xc2x11st order diffracted lights by the second area form predetermined angles in opposite directions with respect to a radial axis crossing at right angles to a track of the information recording medium, and the holographic optical element first area and second area, and 3-beam generating diffraction grating are constituted in such a manner that when one of the xc2x11st order diffracted lights branched by the 3-beam generating diffraction grating is a first side beam, and the other is a second side beam, the diffracted light of the first side beam by the first area is overlapped with the diffracted light of the second side beam by the second area on the light receiving element substrate.
In a preferred embodiment of the present invention, a focus error signal is obtained on the basis of signals from a first light receiving element and a second light receiving element disposed in a first position in which the diffracted light diffracted by the first diffraction area is converged on the light receiving element substrate and a second position in which the diffracted light diffracted by the second diffraction area is converged on the light receiving element substrate, respectively.
In a preferred embodiment of the present invention, the diffracted lights to the first and second positions are both +1st order diffracted lights by the first diffraction area and the second diffraction area, and a tracking error signal is obtained on the basis of signals from the light receiving elements disposed in the third and fourth positions in which xe2x88x921st order diffracted lights by the first diffraction area and the second diffraction area are converged on the light receiving element substrate, and the signals from four light receiving elements, disposed on the light receiving element substrate, for detecting the +1st order diffracted light of the second side beam by the first area, the +1st order diffracted light of the second side beam by the second area, the xe2x88x921st order diffracted light of the second side beam by the first area, and the xe2x88x921st order diffracted light of the second side beam by the second area, respectively.
The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.